Cesium is one of the main contaminants of sodium used as the coolant in a “Na-FNR” type of nuclear reactor. For safety reasons, it is necessary to extract radioactive cesium isotopes to reduce the radiological activity of sodium during or after the reactor's operational phase. For that purpose, the sodium which has been contaminated by cesium is liquefied and then filtered through a cesium trap.
This trap typically comprises a porous structure protected by a cladding. It may for instance be a reticulated vitreous carbon cartridge known as a “RVC cartridge”, as described hereafter.
Although, during the filtration process, the cesium trap retains cesium mainly through adsorption, one of its drawbacks is to also retain some of the sodium in its pores.
The trap then becomes a form of nuclear waste, which presents a dual risk in terms of safety and security:
a chemical risk due to the residual sodium which must be maintained in an inert gas atmosphere (such as argon or nitrogen) so that it does not induce a chemical reaction risk such as, for example, an explosion by contact with water, or ignition with air;
a radiological risk due to the contamination by radioactive isotopes of cesium (in particular cesium 137) which causes this trap to be classified as radioactive waste, thus imposing very strict safety and security levels.
To process such nuclear waste through conventional systems for discarding contaminated waste, the chemical risk should first be suppressed, that is, the residual sodium should be removed from the cesium-containing trap.
However, in the nuclear field, sodium is conventionally treated by means of a hydrolysis reaction (see for example patent application FR 2,598,248). This reaction is most often carried out in a process cell or in an autoclave and mainly comprises reacting sodium in a liquid or solid state with water to produce sodium hydroxide. However, it has the drawback of being difficult to control and of generating liquid waste, which must in turn be processed.
In an attempt to solve these problems, solid sodium can be treated through a carbonation reaction, in which the sodium hydroxide produced by the hydrolysis reaction is thereafter transformed into carbonate by adding carbon dioxide in its gaseous form, according to the following reactions:Na+H2O→NaOH+½H2 CO2+H2OH2CO3 NaOH+H2CO3NaHCO3+H2ONaOH+NaHCO3Na2CO3+H2ONa2CO3+H2CO32NaHCO3 
The carbonation treatment offers the advantage of generating as final waste product carbonate in solid form.
A facility for carrying out a carbonation treatment is described, for example, in patent application FR 2,888,231. In this application, castable sodium is introduced in a liquid state in an enclosure, which comprises a plurality of trays in which sodium solidifies in the form of thin layers before the carbonation reaction is initiated.
Because of the constraint associated with the formation of thin layers, this facility cannot be used for the treatment of poorly accessible sodium, such as the sodium contained in the porous structure of a cesium trap.
In particular, cutting this trap open in order to access the sodium cannot be envisaged, because the aim is on the contrary to restrict handling operations in the presence of a radioactive substance.
Indeed, this substance most often requires working in a containment enclosure under an inert gas, such as in a glove box. However, cutting operations performed therein are delicate because of the inherent difficulty in performing manipulations within such an enclosure. Therefore, it is generally desired to restrict such operations.
Also, such cutting operations may lead to contamination of the cutting tools by the radioactive substance, for instance cesium 137.